Don closing in on Texas; 91L a potential threat to the Lesser Antilles

Tropical storm warnings are flying along the coast of Texas from Brownsville to Matagorda as Tropical Storm Don closes in on the Texas coast. Don remains a disorganized, moderate strength tropical storm, and appears unlikely to cause major damage or bring much-needed drought-busting rains to Texas. A hurricane hunter plane is in Don, and found highest surface winds of 55 mph at 11:06am EDT. Don continues to have trouble with moderate wind shear of 10 - 20 knots, which is injecting dry air to the northwest into the storm. Water vapor satellite images show this region of dry air to the northwest of Don. Visible satellite imagery from this morning shows little change to Don so far today, with most of the heavy thunderstorms on Don's south side, and the cloud pattern elongated, the sign of a storm struggling with wind shear. Radar out of Brownsville, Texas shows the main rain areas are on the south of Don's center, and bands of heavy rain are now very close to the Texas/Mexico border.

Figure 2. The latest drought map for Texas shows that over 75% of the state is in exceptional drought--the highest category of drought. Image credit: U.S. Drought Monitor.

Forecast for DonThe wind shear and dry air affecting Don will continue until landfall, and it is unlikely Don will intensify to more than a 60 mph tropical storm. With most of the heavy thunderstorms displaced to the south side of the storm, Mexico will be the primary beneficiary of Don's expected rains of 3 - 5 inches. South Texas will see modest rains of 1 - 2 inches over a few isolated areas, but Don is going to do very little to bring drought relief to the state. Most of Don's rains in Texas will be concentrated in the extreme southern portion of the state near Brownsville. This portion of Texas is experiencing only moderate drought, and the extreme to exceptional drought areas of the state will miss the bulk of Don's rains.

For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast. The 11 am version of this forecast shows that Corpus Christi Texas has the highest chance of tropical storm-force winds (39+ mph): 48%. The primary threat from Don will be heavy rain, which will caused localized flooding problems. An isolated tornado is also a possible concern.

Figure 3. Morning satellite image of 91L.

African wave 91L a potential threat to the Lesser Antilles A well-organized African wave near 9°N 44°W (Invest 91L) is headed west-northwest at 15 - 20 mph, and could arrive in the vicinity of the Lesser Antilles Islands as early as Monday night. Residents of the Lesser Antilles should pay careful attention to this system, as it has the potential to organize into a tropical storm before reaching the islands. While visible satellite loops currently show only minimal heavy thunderstorm activity and no signs of a surface circulation, there is a pronounced large-scale rotation to the cloud pattern. Water vapor satellite loops show that a large area of dry air from Africa lies just to the north of 91L, and this dry air is inhibiting development. The SHIPS model is diagnosing low shear, 5 - 10 knots, over 91L, but the University of Wisconsin CIMSS analysis shows that moderate shear, 10 - 20 knots, is affecting 91L. Sea surface temperatures are 27.5° - 28°C, which is 1° above the 26.5°C threshold usually needed to support a tropical storm.

Forecast for 91LLow to moderate wind shear of 5 - 15 knots is predicted along 91L's path over the coming three days, which should allow the storm to steadily organize, assuming it can shut out any incursions of dry air that might intrude. The latest 06Z run of the GFS model does show 91L developing into a tropical storm by Monday, but the other three most reliable models for forecasting formation of a tropical storm--the ECMWF, NOGAPS, and UKMET models--show little or no development of 91L in their latest runs. On Monday, when most of the models predict that squalls of rain from 91L will begin affecting the Lesser Antilles, wind shear is expected to rise to the moderate or high range, which should act to interfere with development. The latest runs of the GFDL and HWRF models show 91L developing into a hurricane by Monday, but these models are not to be trusted for systems that have not developed into a tropical depression yet. The long-range path of 91L could take it through the Caribbean or towards the U.S. East Coast; it is too early to know with path might be more probable. NHC is giving a 30% chance that 91L will develop into a tropical depression by Sunday morning.

Iran records its hottest temperature of all-timeOmidieh, Iran and Shoshtar, Iran hit a scorching 52.6°C (126.6°F) on July 27, the hottest temperatures ever recorded in the country. The previous record was 52.5°C (126.5°F) set in Hamidiyeh on August 4, 2001. If confirmed by the Iranian weather service, these readings would rank as the third hottest undisputed temperatures ever measured in Asia. There have been only two Asian readings matching or exceeding Wednesday's new Iranian record of 52.6°C that are undisputed that I am aware of. Both of these occurred last year, and were recognized by their respective country's meteorological services as new official records for their country:

53.5° (128.3°F) in Moenjodaro, Pakistan on May 26, 2010 52.6°C (126.7°F) in Abdaly, Kuwait on June 10, 2010

Iran is the third nation this year to set a new all-time heat record (no nations have set an all-time coldest temperature record this year, or did so in 2010.) The Democratic Republic of the Congo, the world's 12th largest country, set a new all-time extreme heat record on March 8, 2011, when the temperature hit 39.2°C (102.6°F) at M'Pouya. Congo's previous all-time hottest temperature was 39.0°C (102.2°F) at Impfondo on May 14, 2005.

VacationI'm taking some time off today through August 9 for a family celebration and vacation, and don't plan on blogging again until August 10 unless a major hurricane develops. In my absence, Angela Fritz will be handling most of the blogging duties, and she will have a post on the latest for Don this afternoon. Angela is on Pacific time, so her posts will be later in the day than I make them.

A LARGE TROPICAL WAVE ACCOMPANIED BY A WELL-DEFINED LOW PRESSURESYSTEM IS LOCATED ABOUT 1150 MILES EAST-SOUTHEAST OF THE LESSERANTILLES. THIS DISTURBANCE CONTINUES TO SHOW SIGNS OFORGANIZATION...AND ENVIRONMENTAL CONDITIONS ARE CONDUCIVE FORGRADUAL DEVELOPMENT OVER THE NEXT FEW DAYS. THIS SYSTEM HAS AMEDIUM CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURINGTHE NEXT 48 HOURS AS IT MOVES WESTWARD OR WEST-NORTHWESTWARD AT15 TO 20 MPH.

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICALSTORM DON LOCATED ABOUT 120 MILES EAST-NORTHEAST OF BROWNSVILLETEXAS.

A LARGE TROPICAL WAVE ACCOMPANIED BY A WELL-DEFINED LOW PRESSURESYSTEM IS LOCATED ABOUT 1150 MILES EAST-SOUTHEAST OF THE LESSERANTILLES. THIS DISTURBANCE CONTINUES TO SHOW SIGNS OFORGANIZATION...AND ENVIRONMENTAL CONDITIONS ARE CONDUCIVE FORGRADUAL DEVELOPMENT OVER THE NEXT FEW DAYS. THIS SYSTEM HAS AMEDIUM CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURINGTHE NEXT 48 HOURS AS IT MOVES WESTWARD OR WEST-NORTHWESTWARD AT15 TO 20 MPH.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THENEXT 48 HOURS.

I've been making straightline projections of both NHC and ATCF numbers since the storm's crossover from the Caribbean to the Gulf of Mexico. The landfall projections derived from its ATCF positions have never taken TS.Don north of (former)MatagordaIslandAirForceBase. And those derived from consecutive NHC positions led to results too squirrelly (eg a projected landfall in the Yucatan followed by a projected landfall in Houston, etc) to be taken seriously.

I'll be posting a mapping soon, after the 6pmGMT ATCF numbers are released, and ya can take a look.

Here in Barbados the local Met Office have issued a special weather bulletin regarding what is now a "strong tropical wave" that has the potential to stengthen. We should feel its effects late Sunday night into Monday.

I am only here to learn; been coming since '03. I appreciate much the comments here. I rarely ever comment, like to listen and learn. I just like to admit there are still variable we don't understand or know about. We are talking weather!

Quoting metwombly: The models are unreliable for predicting invests and tropical waves. There needs to be a closed circulation and measurable winds for them to work, and even then they change from update to update.

Quoting ProgressivePulse:The "Models" have done absolutely horrible with pre-storms this year, why would 91L be any different. Look at the satellites and observe, there will be a TD under your nose before you know it.

The models are unreliable for predicting invests and tropical waves. There needs to be a closed circulation and measurable winds for them to work, and even then they change from update to update.

Noobie here to the blog. I need your thoughts. I know it's really early on this Invest 91L But....... I'm taking the family to Grand Cayman leaving this Sunday 7/31/11. We are from Kentucky and praying for great weather. First half of next week looks good. We will be leaving the Caymans the following Sunday. Any thoughts on this developing Wave? Am i gambling here still going down?

No you are safe. They do have a weather bureau in the Caymans. Just be sure to watch the weather forcast so you aren't caught off guard. The storm, if it forms should be in PR by next Thursday.